This invention relates to machines for the measurement, the marking and/or the accurate checking of workparts, and notably of workparts of large size.
Machines for coordinate measuring along three directions fulfill one requirement of the industry: any mechanical workpiece must conform to a drawing and, after machining, a check is frequently carried out on the finished part, to ensure that it is indeed in compliance with the numerical data of the drawing.
In fact, all cutting machines include deviation factors which are translated into errors in the dimensions of the finished part. Such errors are due to deformations of the machines, of the workpiece and of the tool, to thermal effects, to the stresses and to lost motions.
The most currently applied remedy consists of machining the parts by repeating the same operation several times, to effect successively a blanking, a pre-finishing and a finishing step. The finishing consists of a simple "stroking" of the metal, that is to say of a machining without substantial applied force thus eliminating factors of inaccuracy due to stresses.
Notwithstanding however, errors persist and precision machining must be followed by checking operations.
These checks are carried out by means of measuring machines, which must give the dimensions of the part (which the cutting machine cannot do) and must have an accuracy very much higher than that of the cutting machines.
Three-dimensional coordinate measuring and marking machines are known comprising a tracer finger or a stylus pin displaceable along three perpendicular axes x, y and z by means of a motion transmitting mechanism and comprising a system for measuring the position of the tracer finger along the three axes. Since these machines include mechanical parts, they remain subject to a certain number of measurement deviation and error factors, even though they do not have to transmit substantial forces. In particular, these machines remain subject to thermal effects and to the presence of backlash or lost motion. In practice, due to the presence of the transmitting mechanism, an accuracy of some hundredths of a millimeter in machines for measuring parts of large dimensions, such as for example, engine crank cases, is not presently exceeded.
It might be thought that the shortcomings of present measuring machines can be avoided by substituting optical measurements for measurement by means of a mechanical linkage. In fact, this solution does not comply fully with industrial requirements in the use of such machines, which should preferably provide direct reading on a rule or on a display device.
A prior art machine (French Pat. No. 2 088 675) for measuring the dimensions of parts includes optical interferometers and a servocircuit for correcting the errors due to yaw of a carriage along a track. The resultant system is complex and it would hardly be possible to extend that type of correction to all mechanical inaccuracies which result in first order errors.
It is an object of the invention to provide an improved machine for measuring the dimensions of workpieces. It is a more specific object to provide a machine which is highly accurate and however uses a mechanical supporting structure for the tracer finger or stylus pin, with the attendant advantage that use is easy since it involves simply bringing this finger or pin to the point on the workpiece whose dimensions have to be determined.
According to the invention, there is provided a machine for measuring the dimensions of a workpiece, comprising a faceplate for receiving the workpiece and a mechanical structure carrying a feeler for movement along axes at right angles by means providing for direct measuring of the amounts of movement of the feeler along said axes. Means are provided for determining by optical deviation measurement the measuring errors due to lost motion and deformation of the components of the mechanical structure along at least one of said axes. The corrections to be applied to the data delivered by the measuring means are computed and may be made automatically for the display to be free of errors.
The means for determining the errors corresponding to the movement along one of the axes may comprise a source, such as a laser, positioned close to the origin of the axis, emitting a cylindrical light beam parallel to the theoretical axis of movement and a detector of the deviations of a member movable along said axis with respect to the theoretical axis, positioned on the movable member.
The invention is also applicable to a cutting machine, the feeler being replaced by a tool and the measuring means by devices for controlling the movement of the tool along a predetermined pattern.
In the case of a machine including, along at least one of said axes, means for direct measurement by a reader, for example an incremental reader, borne by a member movable along said axis, cooperating with a scale borne by said axis and providing the indication of the movements in digital form, said means for determining the errors may comprise an analog-digital converter and a computer which receives the data supplied by the reader and by the analog-digital converter and supplies a correct measurement to a display device.
Anyway the mechanical structure acts as a support only; the direct measuring means gives a first approximation of the amount of movement of the finger, which approximation can be corrected either by the operator, or automatically by an associated electronic system, receiving the results of the deformation or deviation measurement, the deformation being always very small with respect to the movement proper. The optical system may be extremely simple and have a constitution which bears some similarity with that of optical guidance devices used for guiding missiles.
The invention will be better understood from a consideration of the following description of a machine which constitutes a particular embodiment thereof, given by way of non-limiting example, with reference to the accompanying drawings.